Jo that cry is exactly what Finn does, as it escalates and wanes, escalates and wanes, it's not I need you it's I WANT YOU IN HERE NOW or WHERE DO YOU THINK YOU'RE GOING??? It's so so hard when we are so affected by their cries, I get the adrenaline rush and the hair standing on the back of my neck when Finn cries, but honestly he does not need you in there. He is just really really tired, his body is full of cortisol and it's so much easier if you just stay in there and assist him with sleep. He needs to re-learn how to do it on his own.
What about if you put some ear plugs in to mute the sound? Or if you don't have ear plugs put on your iPhone with headphones on a very low volume with your own soothing music. You will still hear him, as we do, but the effect won't be as bad. It will be like Caleb-lite. And therefore it won't get YOU as upset which will help you to do some sleep training with more confidence. If you are really upset and getting angry etc. then Caleb can tell and he won't back down, as JC says above, consistency.
Hugs my friend, it's so hard when they just fall off the wagon like this. It's not supposed to be this way is it???